The brake systems of modern commercial airplanes are hydraulic and include electrically operated brake-metering valves, autobrake shuttle valves, and antiskid valves. The valves are controlled by an antiskid/autobrake control unit that produces and sends control signals to the valves of the brake system in accordance with a software program that receives inputs from various sensors and control sources. Pilot instructions produced by the operation of brake pedals or an autobrake setting control the operation of the brake metering valves and the antiskid valves in a manner designed to decelerate an airplane in a smooth manner. The brake metering valves control brake pressure and the antiskid valves control the release of brake pressure in a manner designed to minimize wheel skidding.
It has been found that some airplanes experience brake shudder at low speeds. Brake shudder is caused by landing gear structure moving fore and aft at a high frequency. This effect, called "gear walk," is a dynamic instability between the landing gear structure and the brakes of the airplane. Gear walk causes the airplane to shudder to the extent that passengers and pilots find it objectionable. In addition to passengers and pilots finding gear walk objectionable, gear walk is also disadvantageous because it causes increased loads on landing gear structure, which may result in premature failure to due to fatigue.
The present invention is directed to modifying a brake system in a manner that substantially reduces gear walk and, thus, the shudder caused by gear walk.